Objectives: A cross-sectional study was conducted to investigate whether working hours mismatch is associated with workaholism. Material and Methods: This study used the data from the 17th wave (2014) of the nationwide Korean Labor and Income Panel Study. Workaholism was evaluated using the Workaholism Analysis Questionnaire. The final study involved 3157 subjects who answered questions regarding both workaholism and working hours mismatch. To identify the association between working hours mismatch and workaholism according to weekly working hours, a stratification analysis was conducted by dividing the number of working hours/week into 3 groups (≤40 h, 41–59 h, and ≥60 h). The odds ratios were calculated using a multiple logistic regression model, which was adjusted for potential confounders. Results: The workers working more hours than desired showed the greatest frequency of workaholism. As regards workaholism, in all weekly working hours groups, the odds ratios of the group working more hours than desired were 4.28, 95% CI: 2.29–7.99 (≥40 h), 2.14, 95% CI: 1.34–3.43 (41–59 h), 3.40, 95% CI: 1.60–7.21 (≤60 h), which were statistically significant compared to the reference (matched) group. There was no statistically significant relationship between working hours and workaholism when stratified according to the mismatch in working hours. Conclusions: The workers’ working hours mismatch can be significantly related to workaholism. Int J Occup Med Environ Health. 2020;33(2):187–94